1. Field of the Invention
The present invention relates to a fiber-reinforced light metal alloy piston for internal combustion engines.
2. Description of the Related Art
It is well known to manufacture internal combustion engine pistons from light metal alloy castings such as aluminum alloys. Since light metal alloys have a larger coefficient of thermal expansion as compared with steel alloys, the skirt section of the light metal alloy piston is subjected to considerable thermal deformation between the cold start condition and the warmed up condition of the engine. If the piston skirt suction is so sized as to provide little clearance between the outer periphery thereof and the inner surface of the cylinder bore during cold start of the engine, then the friction between the piston skirt and the cylinder bore would become prohibitively high when the engine is warmed up, since the piston clearance in the bore is reduced due to thermal expansion of the piston skirt section. Conversely, if the clearance is large enough to avoid the above-mentioned problem, then the engine will generate piston slap to an unacceptable level during cold start of the engine, because of the excessive clearance between the piston skirt and cylinder bore. In order to meet these opposing requirements, it is desirable to suppress thermal expansion of the light metal alloy piston skirt section so that an optimum clearance is maintained regardless of the engine temperature.
One solution known in the art is to thermally isolate the skirt section from the heated piston crown section by means of a plurality of slits extending through the wall of the skirt perpendicular to the longitudinal axis of the piston. These slits communicate the oil ring groove with the inside of the piston and are primarily intended as oil passages serving to direct oil scraped from the surface of the cylinder bore by the oil control ring toward the interior of the piston. These slits have been found to act as a heat dam that prevents the transfer of heat from the piston crown to the skirt section. However, in high-speed high-power engines, the pistons tend to be subjected to increasingly high heat loads. Therefore, in such high power engines, it is desirable to dissipate heat through the piston skirt section, although most of the heat received by the piston crown from the combustion chamber is primarily transferred through piston rings to the engine cylinders. For this reason, the recent trend in high power engines is to reduce or even abolish the heat dam slits located between the piston crown and the skirt section. This causes the temperature of the skirt section to be elevated by 30.degree. C. to 40.degree. C. as compared with conventional non-supercharged engines, resulting in considerable thermal deformation of the skirt section.
Another solution is to provide within the skirt section a steel ring known as a "thermal strut" and having a high tensile strength sufficient to prevent thermal expansion of the piston skirt. The thermal strut is in the form of an insert and is molded within the matrix of the light metal alloy by an insert casting technique. The disadvantage of such a steel thermal strut is that it increases the weight of the piston and, thus, becomes a bar to designing light weight pistons.
It has been proposed, therefore, to use thermal struts made from fiber reinforced light metal alloys, instead of steel thermal struts, as disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) Nos. 59-229033 and 59-229034, and Japanese Unexamined Utility Model Publication (Kokai) Nos. 60-12650, 60-28246, 60-28247, and 60-28248. The thermal strut of fiber reinforced light metal alloys comprises a circumferentially wound bundle of high-tensile-strength inorganic fibers, such as carbon fibers, which are integrally molded within a matrix light metal alloy to form an annular fiber-reinforced portion within the confinement of the shoulder portion of the skirt section. In the fiber reinforced portion, individual fibers are firmly bonded to the matrix metal. Due to the low coefficient of thermal expansion of the high tensile strength fibers, the annular fiber-reinforced portion serves as a thermal strut which precludes thermal expansion of the shoulder portion of the skirt section.
However, the problem which must be overcome in the design of light metal alloy casted pistons having thermal struts comprising high tensile strength carbon fibers is that cracks are formed in the metal matrix of the skirt shoulder portion along the boundary between the fiber reinforced metal portion and the non-reinforced metal matrix portion situated radially outwardly of the fiber reinforced portion, thereby causing breakage of the piston skirt. It is recognized that the formation of cracks is due, in the first place, to the low flexural or bending strength of carbon fibers. Carbon fibers are manufactured by carbonizing acrylic fibers and the like having polymer molecules highly oriented in the longitudinal direction of fibers and present a high tensile strength in the longitudinal direction. However, the shortcoming of carbon fibers as used to form thermal struts is that their resistance against transverse stress is quite insufficient. Thus, when the piston is repeatedly subjected to transverse stress due to explosive pulses imparted thereon during power strokes of the engine or due to thermal expansion and contraction as the piston is repeatedly heated and cooled in response to engine stopping and restarting, carbon fibers tend to break due to their poor flexural strength and the bondage between individual fibers and the matrix metal is lost, thereby leading to crack formation. It is believed that formation of cracks is due, in the second place, to a large difference between the coefficient of linear thermal expansion of carbon fibers and that of the matrix metal alloy. For example, the coefficient of linear expansion of aluminum alloy is on the order of 20.times.10.sup.-6 /.degree. C., whereas that of carbon fibers is about -1.2.times.10.sup.-6 /.degree. C. Therefore, when the piston is repeatedly heated and cooled, the matrix metal located in the non-fiber-reinforced portion adjacent to and radially outward of the fiber reinforced portion undergoes a considerable amount of repeated expansion and contraction, whereas the matrix metal located in the fiber reinforced portion remains substantially free from such expansion because of restraint by reinforcing fibers. As a result, the matrix metal in the non-reinforced portion is subjected to a large stress which gives rise to cracks along the boundary between the fiber reinforced portion and the outer non-reinforced portion.